Benzylguanine is a potent inactivator of alkylguanine transferase, a DNA repair protein that confers resistance to carmustine. This study will evaluate the pharmacokinetics of benzylguanine in man, the time course of alkylguanine transferase inhibition, the optimal dose of BG and the maximum dose of carmustine that can be safely administered with BG in patients with advanced cancer. Phase II studies of the combination of BG and BCNU are ongoing or planned in patients with malignant gliomas, myeloma, colorectal cancer, melanoma, soft tissue sarcoma, and pediatric brain tumors.